Actuators are commonly used in a variety of contexts to control devices. For example, actuators are used in heating, ventilating, and air-conditioning (HVAC) systems to open and close dampers to regulate airflow through ventilation ducts.
A typical actuator includes a spring return to drive a damper coupled to the actuator back to an initial or closed position. The spring return includes a spring that is wound by the actuator's motor as the actuator opens the damper. The energy stored in this spring is used to return the damper to the initial position upon loss of power.
Under less than peak load conditions, the spring of the actuator can cause the actuator to accelerate to high speeds as the damper is returned to the initial position. This is often undesirable because excessive speed can cause damage to the actuator or controlled device. For this reason, some means of controlling the return speed is desirable.
Prior actuator designs to control the acceleration of the actuator caused by the spring return during power failure have added some combination of electrical and mechanical components to the actuator to limit the maximum spring return speed. See, for example, U.S. Pat. Nos. 4,572,333, 4,771,643, 5,182,498, 6,249,100, and 6,369,540.
For example, in U.S. Pat. Nos. 6,249,100 and 6,369,540, a zener diode is placed in series with a conventional diode to regulate the voltage induced across the windings of the motor and to thereby enhance a braking effect provided by the motor in its unenergized state when the motor is rotating in reverse under the force of the spring return.
In another example disclosed in U.S. Pat. No. 4,572,333, a pinion assembly of the actuator includes shoes that move outwardly as the motor increases in rotational speed and frictionally engage an internal drum surface to govern return speed.
However, these designs add cost and only provide for a limit on maximum speed of return. The designs do not allow the actuator return speed to be further reduced when nearing the initial or closed position (i.e. the end stop) to reduce gear train loads when the actuator reaches the end stop. Therefore, other implementations often employ a one-way clutch mechanism to decouple the spinning motor's inertia from the gear train when the motor reaches the end stop. However, these clutch mechanisms also add expense and are an additional wear item in the gear train.
Therefore, it is desirable to provide new systems and methods for regulating a speed of return of a spring return actuator upon power failure.